Basically, I used two pieces of 1/2" plywood, both hinged from the crossmember in the interior. I used crimp-in threaded inserts to mount the hinges, then used cardboard to make templates for the plywood lids.

In back, two sliding latches secure the lid. That lid opens wide enough to replace the car's battery when necessary. I kept the oem padded mat. There's not a lot of room here, but enough for a few tools, jumper cables, etc.

In front, a loop of bungee cord secures the lid. I dynamatted that area, then taped down carpet sections left over after I trimmed the 4x6 carpet mat to fit on top.

I will attach the mat with some carpet tape and a few trim screws.

Just thought some folks might like to see another version of a rear seat delete!